In the field of non-impact printing, the most common types of printers have been the thermal printer and the ink jet printer. When the performance of a non-impact printer is compared with that of an impact printer, one of the problems in the non-impact machine has been the control of the printing operation. As is well known, the impact operation depends upon the movement of impact members such as wires or the like and which are typically moved by means of an electromechanical system which is believed to enable a more precise control of the impact members.
The advent of non-impact printing, as in the case of thermal printing, brought out the fact that the heating cycle must be controlled in a manner to obtain maximum repeated operations. Likewise, the control of ink jet printing in at least one form thereof must deal with rapid starting and stopping movement of the ink fluid from a supply of the fluid. In each case, the precise control of the thermal elements an of the ink droplets is necessary to provide for both correct and high-speed printing.
In the matter of ink jet printing, it may be extremely useful to make certain that a clean printed character results from the ink droplets. Several of the problems which have been encountered relate to the existence of the ink droplet, the position of the droplet, the size of the droplet and the property or condition of the ink spot on the record media.
It is therefore proposed to observe the details of the ink jet printing operation by way of sensing or supervising the depositing of the ink droplets on the paper.
Representative prior art in the field of sensing printed indicia includes U.S. Pat. No. 3,562,761, issued to J. J. Stone on Feb. 9, 1971, which discloses drop phasing in ink drop writing apparatus wherein ink emitted by the nozzle is in the form of ink drops which are charged in a tunnel in response to video signals and means are provided for sensing whether or not the ink drops are made to occur with the proper phase to assume the proper charge and, if this is not true, to correct the phase of the vibration of the nozzle whereby the ink drop phasing and charging are corrected.
U.S. Pat. No. 3,761,941, issued to J. A. Robertson on Sept. 25, 1973, discloses phase control for a drop generating and charging system wherein charged drops pass through an electrical deflection field into a catcher, and drops which are uncharged pass undeflected through the field and onto a recording sheet. During non-recording times, a calibrating signal is applied to the charging electrode and the charge is measured by an electrometer. This measurement indicates the phase of drop generation relative to the phase of the calibrating signal and deviations of this relative phase from a desired phase are corrected by adjusting the amplitude of the drop stimulating disturbance applied to the ink jet.
U.S. Pat. No. 3,810,194, issued to K. Tokunaga et al. on May 7, 1974, discloses a device for generating a pulse in response to a droplet formation or movement to synchronize the operation of a deflection means or a pattern generating means of the printer with the pulse from the droplet detecting means. The ink droplet detecting means has an electroconductive plate and a resistor to which the plate is connected to a voltage supply.
U.S. Pat. No. 3,886,564, issued to H. E. Naylor et al. on May 27, 1975, discloses deflection sensors for ink jet printers positioned in a test location downstream from the nozzle means and in proximity to the test path and positioned for sensor coupling with ink drops proceeding in the test path. The sensor means comprises a pair of sensor plates separated by a reference gap located adjacent the test path wherein passage of ink drops induces signals representative of charges of the drops in the sensor plates due to coupling between drops and the sensor plates.
U.S. Pat. No. 3,977,010, issued to B. T. Erickson et al. on Aug. 24, 1976, discloses a dual sensor for a multi-nozzle ink jet comprising electrically conductive sensing means disposed on opposite sides of the ink drop streams and electrically conductive shielding means disposed fore and aft of the sensing means and current amplification means connected to each sensing means. Measurements are made on a jet stream to sense ink droplet alignment, droplet arrival time, charge electrode operation, and charge phasing.
U.S. Pat. No. 4,129,875, issued to S. Ito et al. on Dec. 12, 1978, discloses phase control for an ink printer which uses a detector circuit for detecting the relation between the generation of an ink droplet and the phase of a charging signal on the basis of an output signal of a sensor. A phase shift circuit matches the generation of the ink droplet and the phase of the charging signal and an inhibit circuit inhibits the phase shift circuit from operating for a predetermined period of time.
And, U.S. Pat. No. 4,176,363, issued to T. Kasahara on Nov. 27, 1979, discloses ink jet printing apparatus wherein the print head is shifted to an ink failure preventive ejection position distal from the printing region and has a timer for generating a signal and a detector for detecting the setting of the print head at the ejection position.